Significant Reduction in Seroprevalence of Antibodies Against Hepatitis A across Thailand, 2024.

Hepatitis A virus (HAV) is an RNA virus that causes acute hepatitis and is transmitted via the fecal-oral route. It has historically been highly endemic in Thailand, where most children develop lifelong immunity after infection. Economic development and improved sanitation have reduced HAV transmission, but immunity levels have declined, raising concerns about potential future outbreaks. This study aims to assess the seroprevalence of HAV antibodies in Thailand in 2024, 10 years after the last national survey in 2014, and to evaluate current immunity levels to inform public health strategies. A cross-sectional study was conducted in a population aged 6 months to 80 years across Thailand's geographic regions. A total of 4,312 serum samples were tested for anti-HAV antibodies using the chemiluminescent microparticle immunoassay. The seroprevalence data were compared with findings from previous surveys in 2004 and 2014. The study showed a significant decline in population immunity to HAV, with the age at which 50% of individuals had antibodies increasing from 36 in 2004 to 42 in 2014, and to 47 years in 2024. A majority of the population remained susceptible to HAV, particularly among younger age groups. Thailand has transitioned to low HAV endemicity, with a large proportion of the population lacking immunity. Despite the absence of significant outbreaks in recent decades, the risk of future outbreaks remains, particularly from imported cases. Enhanced surveillance and vaccination strategies are necessary to prevent future HAV transmission and manage public health risks.